The invention relates to a method for producing crystallized silicon according to the EFG process using a shaping part, between which and a silicon melt crystallized silicon grows in a growth zone, whereby the inert gas and at least water vapor are fed into the silicon melt and/or growth zone, by means of which the oxygen content of the crystallized silicon is increased. The invention further relates to crystallized silicon, which is produced according to the EFG process and contains carbon and oxygen.
Silicon produced according to the EFG (edge-defined film-fed growth) process is crystallized silicon that is obtained by using a capillary slot as a shaper for the melt in the form of thin-walled ribbons or polygonal or cylindrical thin-walled tubes of crystallized silicon. The related method and embodiments thereof are described, for example, in U.S. Pat. No. 5,098,229 or U.S. Pat. No. 6,562,132, as well as in the cited documents.
When crystallized silicon is produced according to the EFG process, silicon is grown in an open-top vacuum chamber with a built-in furnace, bypass devices and, in particular, a crucible to receive the silicon melt. Independent of the concrete embodiment, an inert gas, such as argon, continuously flows through the vacuum chamber at normal pressure. These provisions can prevent and/or minimize air and/or oxygen from entering the vacuum chamber and the associated oxidation of the silicon melt, the hot graphite parts in the vacuum chamber, as well as of the crystallized silicon, like silicon ribbons or silicon tubes.
When producing high-quality silicon by means of the EFG process, it is of crucial importance that the level of the silicon melt, namely the molten pool, is possibly kept stable, so that the thermal conditions in the crystallization area, that is, in the area between the shaping part and silicon melt are kept stable. This is ensured by continuously recharging the silicon granules which can be conveyed and dosed into the crucible in a controlled manner. In this case, control means that the mass of the silicon granules to be recharged corresponds to the mass leaving the melt as a result of the growth of the crystallized silicon, namely the ribbon and/or tube.
Appropriately crystallized silicon is used to a large extent in solar cells. Because of the photovoltaic properties of the silicon grown by the EFG process, it is important that the melt is contained in a graphite crucible, and not in a quartz crucible and/or oxide ceramic crucible, as is the case with other methods, such as the Czochralski crystallization method, according to which a massive crystal is pulled out of the melt, or with directional solidification of the melt in the crucible.
In the EFG process, the melt is not in direct contact with oxidic material, but exclusively with graphite, which then reacts to become silicon carbide in the contact area with the silicon melt. Consequently, in the EFG process the carbon content of the melt inevitably has the order of magnitude of the solubility of carbon in the silicon melt determined by the phase diagram silicon-silicon carbide as well as by the temperature, whereas the oxygen content is comparatively low, typically below 5×1015 cm−3 and thus approx. two orders of magnitude below the silicon produced by means of one of the previously mentioned further crystallization methods.
From U.S. Pat. No. 4,415,401 (DE-C-31 09 059) and several complementary publications, like “B. Mackintosh, J. P. Kalejs, C. T. Ho., F. V. Wald; Multiple EFG silicon ribbon technology as the basis for manufacturing low-cost terrestrial solar cells; Proc. 3rd ECPVSEC, Cannes 1980, 553”, “J. P. Kalejs, L.-Y. Chin; Modeling of ambient meniscus melt interactions associated with carbon and oxygen transport in EFG of silicon ribbon; J. Electrochem Soc. 129 (1982) 1356”; “B. Pivac, V. Borjanovic, I. Kovacevic, B. N. Evtody, E. A. Katz; Comparative studies of EFG poly-Si grown by different procedures; Solar Energy Mat. & Solar Cells 72 (2002) 165”, it is known that the oxygen content of the silicon produced according to the EFG process can be increased beforehand to above 1×1017 cm−3 by the controlled addition of carbon monoxide or carbon dioxide to the inert gas, and that this way a significant increase in the average value of the minority carriers lifetime and/or the degrees of efficiency can accordingly be achieved with solar cells.
The addition of a carbon-containing oxygen source, like CO or CO2 (U.S. Pat. No. 4,415,401) is preferred compared to the addition of air and/or oxygen because an increased oxygen content in the atmosphere of the vacuum chamber may primarily lead to increased burnup of the graphitic components, and hence to quicker destruction thereof. Using air and/or oxygen, the required carbon monoxide, which, according to U.S. Pat. No. 4,415,401, reacts with the silicon melt to become silicon carbide, will not be formed until a corresponding reaction occurs. In this process, the oxygen in the melt will be diluted. Thus, the direct addition of carbon monoxide and/or carbon dioxide reduces the burnup of the furnace components with otherwise the same effect. According to the known publications and taking these considerations into account, with the EFG process, silicon is produced with increased oxygen content by the addition of carbon monoxide. However, the disadvantage of the related method is that the proportion of silicon carbide and/or carbon in the melt increases together with the oxygen concentration, which may result in the properties of the crystallized silicon deteriorating, especially when used in solar cells.
According to U.S. Pat. No. 4,415,401 (DE-A-31 09 51), the melting zone should be fed with inert gas of the highest available purity. If need be, a maximum of between 10 and 25 ppm of water vapor as impurity may be contained.
DE-B-1 291 322 refers to a method for growing a semiconductor crystal with differently doped zones. In order to achieve the differently doped zones, an oxidizing gas flow is used.
According to JP-A-05 279 187, TiO2 monocrystals can be grown with the EFG process. In order to achieve stabilization of the crystal structure, H2O can be added in a proportion of 1% to the inert gas containing oxygen that flushes the melting zone.
The citation Pivac, B; Borjanovic, V; Kovacevic, I; Evtody, B N; Katz, E A; “Comparative studies of EFG poly-Si grown by different procedures”, Solar Energy Mat. & Solar Cells 72 (202), pp. 165-171, deals with impurities in polycrystalline silicon, namely depending on the methods eligible for application, including the Czochralski method, float zone (FZ) method and EFG method.
In the citation Kalejs J P: “Point defect, carbon and oxygen complexing in polycrystalline silicon”, Journal of Crystal Growth 128 (1993), pages 298-303, the theory of the occurrence of point defects as a dependency of the relation between the growth velocity and temperature gradient known from crystallization of monocrystalline, dislocation-free silicon according to the Czochralski method and float zone (FZ) method is transferred to EFG conditions.
EP-A-0 54 656 refers to a method for crucible-free zone melting.
The subject matter of DE-C-33 17 954 is a semiconductor element with a semiconductor layer of polycrystalline silicon film.